This invention relates to improvements in load-handling attachments for lift trucks wherein selectively openable and closable opposed load arms (i.e. clamp arms, laterally positionable forks or other load-engaging implements) are attached to the ends of transversely oriented, horizontally movable elongate slide members mounted on a slide guide frame attached to a lift truck load carriage.
In the past, stress concentrations have been responsible for a high incidence of fatigue failures at the joint between the rear of a forwardly protruding load arm and the end of the respective slide member upon which the load arm is mounted. Such failures result from the cyclic application of primarily horizontal forces to the joint, caused by the load moment and clamping moment respectively. The load moment is the reactive moment exerted about a generally horizontal axis transverse of the lift truck by the weight of the forwardly protruding load, causing the load arm to pull forwardly away from the truck at its top and push rearwardly toward the truck at its bottom. The clamping moment is the reactive moment exerted about a generally vertical axis by the clamping pressure on the load which tends to pivot the load arm laterally outward with respect to the end of the respective slide member upon which it is mounted.
In conventional structures utilizing tube-type slide guides, as shown for example in Ehmann U.S. Pat. No. RE 23,694, each slide member includes a metal connecting member of small cross section and length, compared to the cross section and length of the remainder of the slide member, protruding forwardly at one end of the slide member and attaching at its forward end to the rear of a load arm. The relatively small cross section and length, and the forward protrusion, of the connecting member is necessary because such member must communicate between the interior and exterior of the tube-type slide guide, through a narrow slot in the guide, when the load arms are closed to a narrow position within the width of the clamp guide frame. This type of construction, which is also utilized in structures having tube-type guides of square or rectangular shape as shown for example in U.S. Pat. Nos. 2,609,114 and 2,746,630, introduces a high degree of susceptibility to failure from the above-described cyclically applied horizontal forces because the protruding connecting member at the end of each slide member causes a relatively abrupt cross-sectional change horizontally in the slide member at the point where the connecting member protrudes, thereby creating a stress concentration at that point. Moreover, the forwardly protruding nature of such connecting member provides an additional lever arm through which the horizontal reactive clamping forces can act on the stress concentration point, thereby maximizing the size of the clamping moment acting on the stress concentration point.
Some previous clamps have sought to remedy the above-described susceptibility to fatigue failure from cyclic horizontal forces by providing large horizontally extending gussets at the juncture between the slide member and load arm, as shown for example in U.S. Pat. Nos. 2,635,774, 2,870,929 and 2,956,700. However such gussets interfere with the handling of a rectangular load in the optimum position where the rear surface of the load is closely adjacent to the slide guide frame. Attempting to hold a rectangular load in a more forward position where it does not interfere with the gussets is unsatisfactory since the greater forward tipping moment thus exerted by the load on the lift truck greatly reduces the load-carrying capacity and stability of the truck.
Still other clamp constructions have attempted to solve the aforementioned failure problem by providing slide members of uniform cross section throughout their length, each having an elongate portion along the front thereof which is exterior of the slide guide and to which the load arm may attach directly without the necessity of any protruding connecting member. In one such construction, the tube-type guide and slide construction previously described is reversed such that the tube serves as the movable slide member and is slidably mounted over an interior guide on the slide guide frame. In another analogous construction, horizontally oriented channels having open fronts are utilized as the guides, the slides constituting horizontally oriented, elongate I-beams having their rear flanges riding within the respective channels and their front flanges projecting forwardly and exterior of the front of the channels. In these constructions, where the slide members each contain a longitudinal front portion extending along the length thereof which is exterior to the guide, there is no need for a forwardly protruding connecting member at the end of the slide member for attachment to the load arm, but rather the load arm may be connected directly to the longitudinal front portion of the slide member thereby eliminating any abrupt horizontal change in cross section of the slide member and eliminating the additional lever arm for clamping forces characteristic of those clamps having tube-type slotted guides.
Unfortunately, although the latter clamp constructions improve the resistance to failure of the joint between the end of the slide member and rear of the load arm, they also maximize the overall vertical height of the clamp since the large front portions of the respective slide members occupy a common vertical plane and must pass one another in that plane when the clamp is closed to a narrow position. Accordingly the slide members must be spaced apart vertically sufficiently such that no part of their respective front portions which occupy the same vertical plane overlap vertically, which would otherwise cause interference between adjacent slide members when the clamp is closed to a narrow position. Such vertical spacing makes it very difficult to achieve a clamp which also has a low vertical height; however low vertical height is extremely important when loading to the ceiling in a boxcar or other low ceiling area where the clamp guide frame frequently strikes the ceiling before the load can be lifted into place.
Accordingly, what is needed is a slide structure which minimizes the aforementioned stress concentration and susceptibility to failure at the joint between the load arm and the slide, but which is also capable of producing a minimal overall vertical height of the clamp structure.